In certain situations and circumstances, it is important to know if the insulation surrounding a wire or cable has deteriorated. Such deterioration could be a precursor to a failure in an important system. For example, in jet fighter aircraft, wire chaffing and the resulting deterioration of the insulation are often precursors to failure. If the chaffing continues undetected, the cable may be severed or shorted to another cable with catastrophic results depending on the purpose of the cable. If the cable is used to control the rudder or aileron, severing or shorting of that cable could result in a loss of control of the aircraft and possibly a fatal crash. It would be desirable therefore to be able to detect deterioration of wire insulation before failure occurs. If insulation defects and deterioration are detected in a timely manner, the wire or cable could be replaced before a catastrophic failure occurs.
U.S. Pat. No. 3,096,478 discloses an apparatus for detecting non-uniformity in electrically insulated wires through the use of conductive gas electrodes. The electrodes consist of a tube or sleeve containing ionized air which establishes a direct current path through the defective insulation segment. In the apparatus disclosed in this patent, the cable must be placed inside the conductive gas electrode. The conductor of the cable itself must be grounded and the cable must be moved through the conductive gas electrode during the test, so that the system can be employed essentially only for testing cables prior to installation. A current or voltage of fixed magnitude as applied to the, or one, electrode and a fault indication is based on a change in current flow through the cable insulation.
U.S. Pat. No. 3,263,165 describes apparatus for detecting corona-producing cable insulation defects. For this purpose, a voltage gradient is established along the cable by electrodes containing an ionized gas, the voltage being sufficient to cause gaseous areas in the cable insulation to become ionized and produce high frequency radiation which is detected. The cable is advanced through the apparatus and does not itself conduct any signal. Here again, the cable must be tested prior to installation.
The systems disclosed in both of these references require the measurement of very small currents in the cable under test and require that the cable not be in an installed state. These requirements considerably limit the range of applications of such systems.
U.S. Pat. No. 3,639,831 discloses a method and apparatus for producing a directable, electrically conducting gas jet and detecting the presence of anomalies therein caused by insulators, conductors or semiconductors. The gas jet flows across a test zone and impinges upon a target anode which is maintained at a bias potential with respect to the cathode of the ionizing generator such as to cause an electrical current to flow between the anode and the cathode via the gas jet across the test zone. This device requires that the electrical current flow between the gas jet nozzle and the target anode be constant. Moreover, there is no provision for measuring anomalies which have their own potential or are carrying a current.
Allowed U.S. application Ser. No. 07/267,138, filed on Nov. 4, 1988, discloses devices for detecting cable insulation defects which utilize an ionized gas stream or cloud in a manner which differs fundamentally from the techniques described in the above-cited patents. These devices can be used to test installed cables which are connected in an electrical system and are carrying their normal signals.
The devices disclosed in application Ser. No. 07/267,138 include an ion source and at least one sense conductor, both disposed adjacent the cable to be tested while the latter is carrying a normal alternating voltage. According to one embodiment, an ion source is moved along the cable under test to produce a localized ion cloud forming a conductive region between the cable and a single sense conductor. When a cable insulation defect of sufficient severity is encountered by the ion cloud, the electric field produced by the signal on the cable conductor induces a corresponding signal on the sense conductor. This occurrence can be detected by performing synchronous detection of the sense wire signal with respect to the cable conductor signal. The location of an insulation defect can be determined by noting the position of the ion source at the moment when a synchronous detection output signal is being produced.
In a copending application filed concurrently with the present application, entitled DEFECT POSITION LOCATOR FOR CABLE INSULATION MONITORING, and bearing Westinghouse Case No. 55,431, there is disclosed a device which operates according to the principles disclosed in the above-cited pending application and which employs coded position conductors to which respective signals are applied. These signals will be coupled into the sense wire and then synchronously detected together with the sense wire current signal to provide an indication of the instantaneous location of the ion cloud along the length of the position conductors.
In the operation of devices employing a bare sense wire, the sense wire acts as a capacitive pick-up probe in which currents will develop in response to surrounding electromagnetic fields, including those produced by extraneous electrical systems such as power lines, even in the absence of an ion cloud. The presence of an ion cloud increases this coupling. In addition, in the presence of an insulation defect, or a bare wire carrying a signal, at a location enveloped by an ion cloud, the sense wire is resistively coupled to these signals, resulting in the production of current components in the sense wire which are in phase with the signals on the cable conductor or other bare conductors. However, these in-phase currents, which it is desired to sense, are quite small in comparison to large capacitively coupled currents, such as those produced by nearby power lines. Because of the low level of these in-phase currents, which are the currents to be detected, it may be difficult to separate them from the other current components generated in the sense wire.
Moreover, in devices of the type here under consideration, variations can occur in the distance between the sense wire and the cable conductor under test, as would occur particularly when use is made of a portable ion source carrying the sense wire. When this occurs, the currents induced in the sense wire will vary as a function of such distance variations. In addition, the currents induced in the sense wire will vary as a function of the strength of the ion cloud. These variations can be sufficiently large to effectively mask out the current variations resulting from cable insulation defects.